wibrary

I heard someone from Wisconsin who constantly pronounce "library" as "wibrary". Why is this?

They must have had a speech impediment. It's odd though, as there was someone I heard who claimed to be from Wisconsin who had the exact same speech problem.

It might not be a speech impediment. It might be a Wisconsin dialect.

That is a dialect feature here in Wisconsin, where it is very common to realize /l/ as [M\] or [M], including word-initially. It is not a speech impediment at all; for instance, I commonly use such a pronunciation when speaking English even though when speaking German I use [l] for German /l/.